This invention relates to in situ recovery of shale oil and, more particularly, to techniques for attenuating airblast produced when detonating large amounts of explosive for forming an in situ oil shale retort.
The term "oil shale" as used in the industry is in fact a misnomer; it is niether shale, nor does it contain oil. It is a sedimentary formation comprising marlstone deposit with layers containing an organic polymer called "kerogen" which, upon heating, decomposes to produce hydrocarbon liquid and gaseous products. The formation containing kerogen is called "oil shale" herein, and the hydrocarbon liquid product is called "shale oil".
One method for recovering shale oil is to form an in situ retort in a subterranean formation containing oil shale. Oil shale formation within an in situ retort site is fragmented to form a retort containing a fragmented permeable mass of formation particles containing oil shale. The formation particles at the top of the fragmented mass are ignited to form a combustion zone, and an oxygen-supplying gas, such as air, is supplied to the top of the fragmented mass to sustain the combustion zone and to advance the combustion zone downwardly through the fragmented mass. As the combustion zone advances through the fragmented mass, heated retorting gas forms a retorting zone on the advancing side of the combustion zone where kerogen in the formation particles is decomposed to produce shale oil and product gases. Thus, a retorting zone moves from top to bottom of the fragmented mass in advance of the combustion zone. The shale oil and product gases produced in the retorting zone pass to the bottom of the fragmented mass for collection.
U.S. Pat. No. 4,043,595, which is assigned to the same assignee as this application, discloses a method for explosively expanding formation containing oil shale to form an in situ oil shale retort. That patent is incorporated herein by this reference. According to a method disclosed in that patent, formation is excavated to form a columnar void bounded by unfragmented formation having a vertically extending free face, drilling blasting holes adjacent the columnar void and parallel to the free face, loading the blasting holes with explosive, and detonating the explosive in a single round. This expands the formation adjacent the columnar void toward the free face so that fragmented formation particles occupy the columnar void and the space in the in situ retort site originally occupied by the expanded shale prior to such explosive expansion. A room having a horizontal cross-section that coincides approximately with the horizontal cross-section of the retort to be formed is excavated so as to intersect the columnar void. The blasting holes are drilled and loaded with explosive from the room. In one embodiment, the columnar void is cylindrical and the blasting holes are arranged in concentric rings around the columnar voil. In another embodiment, the columnar void is a slot having large, parallel, planar vertical free faces toward which the formation in the retort site can be explosively expanded. The blasting holes are arranged in planes parallel to such free faces.
Explosive in such blasting holes is detonated in a time delay sequence so that unfragmented formation within the retort site is explosively expanded in segments progressing away from the free face provided by the columnar void. The sequence of blasting is rapid, and in an embodiment disclosed in U.S. Pat. No. 4,043,595, the time delays for explosively expanding formation toward the columnar void span a time period of less than 700 milliseconds. Shorter time delays can be used in other embodiments. In one embodiment, as much as 85 tons of explosive are detonated in a single round for explosively expanding formation toward a columnar void. This produces a powerful explosion which generates a large volume of gas and a resultant airblast which can travel through underground workings leading away from the blasting site. Airblast from such a powerful explosion can cause serious damage to equipment and injury to personnel in such underground workings. Equipment which can be damaged from such airblast cannot necessarily be easily or economically removed from underground workings prior to such explosive expansion. Thus, there is a need to attenuate airblast produced when detonating large amounts of explosive for forming an in situ oil shale retort.